Investors poured more than $73 billion into U.S.-listed ETFs in the first half of this year, roughly two-thirds of it into equities, amid renewed optimism that the U.S. economic recovery remains on a steady, if slow, trajectory of improvement more than five years after the collapse of Lehman Brothers.

A slew of funds from low-cost provider Vanguard, the No. 3 ETF provider by assets, dominated the list of the most popular ETFs in the first half­—a fitting cluster, given that the Malvern, Pa.-based firm hauled in more assets than any other ETF sponsor. But Schwab also got traction with its lineup of low-cost ETFs, and First Trust continued to gain momentum with its active ETFs and quasi-active "AlphaDex" ETFs.

Total U.S.-listed ETF assets are now at $1.860 trillion—almost 7 percent higher than at the end of last year and more than 26 percent higher than at the end of 2013's first half. The asset growth reflects a nearly 6 percent increase in the S&P 500 Index.

ETF asset gathering is nowhere near the pace necessary to match the one-year inflows record of $188.54 billion set last year, according to data compiled by ETF.com. The asset growth reflects a nearly 6 percent increase in the S&P 500 Index.

That said, last year's first-half asset haul was also about $73 billion, meaning that, in order to reach the record, inflows had to accelerate significantly in the second half of the year—and did. That could happen again. Some analysts have said it is likely to happen, and, in fact, seems to already happening. In June inflows into ETFs topped $25 billion, according to monthly data we compile.

While a risk-on sensibility favoring equities gained momentum once the winter-related swoon ran its course, about 31 percent of all the first-half inflows—or almost $23 billion—were into fixed-income strategies.

That's because the U.S. recovery remains stubbornly choppy; Japan appears committed to weakening the yen by keeping bond yields low; and, not least, the European Central Bank is concerned that strong disinflationary pressures in the eurozone could turn into deflation.